The most exciting thing was discovering live, healthy corals on reefs already as hot as the ocean is likely to get 100 years from now," Stephen Palumbi of Stanford University tells New Scientist.

In the past few years, biologists have discovered that some zooxanthellae - the tiny symbiotic algae that privide the coral with food in return for a home - can live at warmer temperatures than others, making the corals that host them naturally heat-resistant.

What's more, during a heatwave on the Great Barrier Reef in 2006, an Australian team found that many corals that survived the hot period had swapped their algae for more heat-resistant ones.

Algae 2.0
To see how widespread this algae upgrading is, the researchers from Stanford sampled coral colonies from tidal pools that are naturally at different temperatures on the island of Ofu in American Samoa.

They found that the proportion of corals that hosted heat-tolerant algae was directly related to how hot the pools were, suggesting that they are able to adapt to their local conditions.